On Friday, October 21, 6 students from the current and previous two years
of 2.009 and the course instructor went to San Francisco to burn a real
boat with the MythBusters. October 21 was used for final mirror fabrication
and to setup the stands for the array.

On Saturday, October 22, the weather was good. We were
able to cause charring and smoldering in a 1-2 foot wide swath along
much of the boat's length. After three passes over the boat, the hull
was penetrated and a small open flame was achieved. This burning area
was self-sustaining and, about 2 hours after the test had ended, it
was still burning with intermittent flame and had created a roughly
10 inch diameter
hole in the hull of the boat.

We did not, however, achieve a large flash ignition as we did in
the MIT test. We believe that
this was due to the high moisture
content of the boat that was used as a target. In my view, it
is an open question as to whether the boat was representative of a
seaworthy Roman ship... but at least this boat should not be considered
as too easy a target!

The test employed 300, 1 foot square, bronze mirror
tiles (510 phosphor bronze alloy). The number of mirrors was determined
using the first MIT test as
a benchmark and accounting for the different mirror material, along
with the different conditions in San Francisco.

The tiles were polished using Brasso by the MythBuster and MIT team.
The polish was quite high on most mirrors. About 30% of the mirrors
had a variety of flatness defects that would reduce their performance
to some degree.

The tiles were aimed in a manner similar to the MIT test, with the
exception that there were 4 tiers of mirrors instead of just two rows.
The mirror array was about 110 feet long, similar to the MIT test.
The goal was to make an array that was big enough to start a fire,
but small enough to implement within reasonable budgetary and time
constraints (my guesstimate is ~3-4 person weeks, mostly spent on
polishing the bronze).

Our understanding was that the target boat was a 30
foot, 1924 wooden fishing boat. It was purchased from its prior owner
after it had sunk and then been retrieved and brought back to port.
We were told that it was much lower in the water before the engine
and other heavy items were removed a few days before the test.

It is believed that the boat planks were made of douglas fir. The
planking was estimated to be 2-3 inches thick, dependent upon location.
The boat had been partially stripped of paint and then coated with
pitch.

If you are interested in what a more authenic ship would be like (both
in design and condition) please review
this slideshow. For estimates of how both moisture content and wood
type would affect ignition time, please see
the FAQ.

The boat was positioned so that it was roughly 150 feet from the center
of the array. Since the array was in a straight line along the edge of a
dock, the furthest mirrors (at the ends of the array) were roughly 160 feet
from the boat.

The test (Saturday, October 22)

The test began at roughly 12:15 and the 300 bronze
mirrors took 10 minutes to aim (14 people working in pairs) using
an aiming process similar to the MIT experiment. The aiming mirror
was initially pointed at the stern of the boat since the motion of
the the earth around the sun caused the point projected by the fixed
mirrors to move towards the bow of the boat over time.

Adam was an awesome mirror aiming machine.

Almost immediately after uncovering the mirrors, a considerable amount
of smoke was visible. Black charring was visible on the side of the
boat in a few minutes. The bright spot was estimated to be 4-5 feet
in diameter... not as tightly focused as we had anticipated based
on preliminary tests, but not terrible either.

As the sun spot moved along the side of the boat (at about 9 inches
per minute), it created a roughly 1-2 foot wide charred band along
the side of the boat, but without open flame. We believe that a significant
amount of moisture was being boiled out of the wood.

When the sun spot had moved to 3/4 along the length of the boat (leaving an
estimated 20 feet long x 1-2 feet wide charred strip on the boat), we attempted
to continually adjust the position of the mirrors to keep the bright spot
on a single location, giving the wood more time to dry and heat. In doing
so, we were not able to keep a focused beam, and the bright spot became much
larger, reducing the power per square foot. Thus, the effect of the beam was
reduced.

At roughly 12:40, we repeated the process, again using
the bronze mirrors. The targeting mirror was aimed at the stern of the
boat and the entire array was re-aimed. This allowed us to make a second
pass over the band partially dried and charred in the first attempt.
There was an even more substantial amount of smoke and the charring
deepened. The beam was better focused than the first attempt (~3-4 feet
in diameter). There were glowing, smoldering embers in a number of locations
along the side of the boat, and in some locations there was white ash
on the surface of the planks... but no open flame. There was a modest
sea breeze at the time.

After letting the beam traverse the length of the boat it was requested
that we try to ignite a sail, again using the bronze mirrors. This created
a bright spot on the sail and no charring whatsoever. The dominant reason
for this is probably that the sea breeze was rising and the thin sail
had substantial convective cooling on both sides of the cloth (see
FAQ xii). Also, the lighter color of the sail may be an important
factor.

During the time that the beam was focused on the sail, charred areas on
the side of the boat continued to smolder. After the test with the sail,
we decided to switch to silver/glass mirrors to see if they could take us
from smoldering embers to open flame. The time was around 1:30 PM (estimating).

The silver mirrors provided a tighter beam that we could more easily focus
(~2-3 feet in diameter). We repeated the process, making a third pass along
the side of the boat, starting with the aiming beam at the stern of the
boat, and letting the bright spot move its way along the side of the boat
(again, at roughly 9-10 inches per minute due to the motion of the earth).
There was substantial smoke, and at one point near midship, there was a
period of near-calm wind. The smoke pattern looked very much like what we
saw just prior to flash ignition in the test at MIT. However, no open flame
appeared, even though there were substantial glowing embers.

At this point, we decided to bring the boat closer
for one last attempt to achieve open flames. The rationale was that,
at a shorter distance, we would be able to more tightly focus the
beam. The boat was brought to a distance of 75 feet. The time was
roughly 2:00 PM.

In hindsight, this was a poor decision on my behalf. It would have
been better to use the time to just make another pass over the boat
since, as became apparent later, the coals were on the verge of reaching
an open flame (the glowing coals can be seen right under Jamie's feet
in the picture at left, which was taken while the boat was being repositioned).

You may want to check the close
up image. If we had applied the sunlight to these areas once more, we
probably would have achieved a much more substantial flame.

Regardless, we brought the boat to a distance of 75
feet and began the aiming process once again, starting from the stern
using the silver mirrors. However, while the beam was still near the
stern, a modest open flame broke out mostly on the inside of the hull
at midship, near a scupper hole. The sea breeze was really picking
up and presumably fanned the burning embers to obtain an open flame.

The open flame was caused by the efforts at 150 feet since the beam
at the closer distance had not yet reached this point on the boat
(Jamie pointed this out to us when the flame broke out). It is not
clear if the switch to silver mirrors was necessary. My opinion is
that the switch was not required.

Before the closer range beam reached the burning area on the boat,
a substantial wind gust blew over several of the mirrors, creating
a rather spectacular effect and ending the experiment. We had modeled
that this would occur when the winds reached 14-15 knots. We did not
have the instrumentation needed to obtain a wind speed measurement.
Thus, we could not make a comparison with our estimated value. We
had planned to finish before 2 PM and thereby avoid the afternoon
period when the sea breeze becomes quite strong.

The fire at midship continued to smolder with glowing
embers and also intermittently burn with open flame. Open flame appeared
to be dependent on fanning by the wind. In the picture on the right
you can see a very small flame (you may want to look at the large
image).

About 2 hours after ending the test, the fire was still smoldering with
intermittent flame. There was a roughly 10 inch diameter hole in the side
of the ship. The fire was extinguished with water.

Final thoughts:
We were able to cause charring and smoldering along the length of the ship.
After three passes along the boat, we were able to penetrate the hull in
one location and achieve an open flame. This burning area was self-sustaining
as a smouldering fire, and about 2 hours after the test had ended it was
still burning and had created a roughly 10 inch hole in the boat.

We were not able to achieve the large flash ignition as we did with the
2.009 experiment at MIT. Most likely, the difference was the moisture content
of the wood, which was very high. Certainly wood in any boat on the water
would have higher moisture content than kiln dried lumber but, given the
history of the boat, it may still be an open question as to whether the
moisture content (above the water line) was representative of a typical
wooden boat (or Roman ship) in good seaworthy condition. In hindsight, it
would have been prudent to have taken a moisture content reading so that
this issue could have been addressed.

All said, we feel that the power estimates and models used to design the
array were quite close to the actual performance, but we did underestimate
the effect of moisture content.

For larger distances, one would want to improve our design to allow a fixed
spot on the boat to be tracked... and inevitably, some fairly simple ways
to do this have already come to mind! Too much fun...